In recent years, with the advancement of semiconductor technologies, portable electronic products and flat panel displays are developed rapidly. Among various flat panel displays, liquid crystal displays (LCDs) free of harmful radiation and characterized by low operating voltage, light weight, and small volume have become the mainstream display products of the market. Generally speaking, since the LCD panel has no luminescent function itself, a backlight module is required to be disposed under the LCD panel, so as to provide a backlight source to the LCD panel.
The conventional backlight modules are broadly divided into cold cathode fluorescent lamp (CCFL) backlight modules and light emitting diode (LED) backlight modules, in which the LED backlight module can improve a color gamut of the LCD, and thus the current panel manufacturers generally employ the LED backlight modules to replace the CCFL backlight modules.
The LED backlight module has a plurality of LED strings arranged in parallel, and each of the LED strings is composed of a plurality of LEDs connected in series. In an actual application, all the LED strings can be operated under a system voltage (VBUS) generated by a boost unit, so as to maintain the same constant current for the current flowing through each of the LED strings.
On the other hand, in some applications, there might be requirements for adjusting brightness because of being cooperated with ambient light or the displayed frames being different. At this current stage, the most commonly used method is to provide a dimming signal in order to simultaneously control an on-off time ratio of the current flowing through each of the LED strings and to achieve the purpose of dimming through using persistence of vision. However, such method causes the momentary load of the boost unit for providing the system voltage (VBUS) to be intensified when the provided dimming signal is enabled, and there will be no load existed when the dimming signal is disabled. By this way, the following three issues are derived:
1. the ripple of the system voltage (VBUS) provided by the boost unit will be increased, thereby causing the instability of the current flowing through each of the LED strings;
2. the voltage conversion ratio of the boost unit is deteriorated in response to the intensified momentary load of the boost unit; and
3. the phenomenon of higher electromagnetic interference (EMI) will be occurred in response to the large current caused by the intensified momentary load of the boost unit.